Significant variations in the twinning rate can be observed both in the long term and in the short term. These may be explained under the hypothesis that women who bear dizygotic twins tend to conceive more readily than other women, and the difference in waiting time to conception has been estimated from national statistics to be about two months. If confirmed, the hypothesis would have implications for the physiology of reproduction. Data on about 50,000 pregnancies collected prospectively in the Collaborative Perinatal Project will permit a direct test of the hypothesis and more precise estimation of any difference in average waiting time. The mothers of dizygotic twins will be compared with matched controls with respect to fertility and birth intervals. This analysis is now approaching completion.